This invention relates to semi-flexible, shock-absorbing polyurethane foams. More particularly, this invention relates to the above type foams containing an amylaceous material and having an open cell structure and a process for preparing same.
Semi-flexible or semi-rigid polyurethane foams are used for various applications. One specific area of application is in automobiles as crash padding for instrument panels, trim, sun visors, horn buttons, arm rests, and the like. Foams for these applications typically are prepared by reacting an organic polyisocyanate with a polyether in the presence of a blowing agent, a catalyst, and, optionally, in the presence of cross linking agents, surfactants, plasticizers, stabilizers, dyes, fillers and pigments. The articles themselves usually are produced by vacuum forming an embossed, flexible thermoplastic sheet, e.g. an acrylonitrile-butadiene-styrene copolymer (ABS) or polyvinyl chloride, to the inside surface of a mold, pouring the mixed foam ingredients into the mold on top of the sheet, and then closing the mold and allowing the foaming reaction to proceed.
Foams used for this purpose preferably should have a high modulus per unit density in order to provide the desired shock absorption at a minimum density, have an open cell structure so as to prevent shrinking during post-cure cooling, and be resistant to deterioration upon aging and exposure to moisture, oxygen, etc. Of course, a reduction in the production cost for these foams is highly desirable.
The use of low cost starches in various types of polyurethane foams including semi-flexible or semi-rigid foams has been proposed. The direct addition of starch along with the other conventional foam ingredients, such as in a "one-shot" process, generally has not been satisfactory because the foams have poor physical properties; the most undesirable of which is a closed cell structure with an attendant undesirable shrinking during post-cure cooling. Such shrinkage usually results in the final molded product having out-of-tolerance dimensions for the intended use.
Examples of prior art processes employing starches in polyurethane foams include U.S. Pat. No. 2,908,657 (Boggs) which discloses the incorporation of various starches into a polyurethane foam by adding the starch to a completely reacted liquid reaction product of a polyester and a diisocyanate. Boggs teaches it is essential that the polyester and diisocyanate be mixed and completely reacted before the starch is incorporated; otherwise, the resultant foam product has poor tear resistance and tensile strength. U.S. Pat. No. 3,004,934 (Dosmann et al) discloses the addition of starches to a liquid prepolymer formed by reacting an organic polyisocyanate with a polyether or a polyester to produce semi-flexible foams having predominantly closed cells. German Patent Application No. 2,448,216, published Apr. 10, 1975, discloses the use of a carbohydrate filler, such as a starch, in polyurethane foams employing a polysicocyanate having a relatively high functionality. Applicant has found that foams including an amylaceous material, such as a cereal starch or flour, and prepared by a "one-shot" process have a closed cell structure, with an attendant undesirable shrinking during post-cure cooling, when an organic polyisocyanate having a functionality in the order of 2.6 to 2.7 is used.
Prior attempts to overcome the difficulties associated with incorporating starches directly into polyurethane foams include using oxyalkylated starches, using polyhydroxypolyoxyalkylene ethers formed by reacting starch with a polyhydric alcohol in the presence of an acid catalyst and then oxyalkylating the resulting reaction mixture as disclosed in U.S. Pat. Nos. 3,227,213 (Fuzesi) and 3,402,170 (Fuzesi et al), and using alkoxylated starch hydrolysates as disclosed in U.S. Pat. No. 3,600,338 (Molotsky).
U.S. Pat. Nos. 3,165,508 (Otey et al), 3,405,080 (Otey et al), and 3,655,590 (Moss et al) disclose the use of starch-based polyols in the production of rigid and flexible polyurethane foams. Attention is also directed to U.S. Pat. Nos. 3,674,717 (Fuzesi et al) and 3,957,702 (Molotsky et al) which disclose the use of phosphorous derivatives of starch polyethers or starch polyether hydrolysates to produce flame retardant polyurethane foams. An article entitled "Rigid Urethane Foam Extended with Starch," in the Journal of Cellular Plastics, August 1967, discloses that rigid polyurethane foams containing starch and made with conventional polyethers do not have acceptable physical properties.